1. Field of the Invention
The present invention relates to a vitreous silica crucible and a method of manufacturing the same, and more particularly, to a vitreous silica crucible for pulling silicon single crystal and a method of manufacturing the same.
2. Description of Related Art
In general, the Czochralski (CZ) method is widely used as a method of fabricating silicon single crystal. According to the CZ method, as shown in FIGS. 1 and 2, a monocrystalline seed crystal 102 is first dipped into silicon melt 101 in a vitreous silica crucible 100. The silicon melt 101 is obtained from polycrystalline silicon. At this point, the seed crystal 102 receives drastic thermal shock, and thus a dislocation is formed at a tip portion of the seed crystal. To remove the dislocation, a neck portion 103 is formed using a predetermined method, such that the dislocation is not transferred to silicon that grows thereafter. Next, a shoulder portion 104 is formed by gradually increasing the diameter of the seed crystal 102 by rotating and slowly pulling the seed crystal 102 while controlling a speed of the pulling and the temperature of the silicon melt. When the diameter of the seed crystal 102 reaches a desired diameter, the seed crystal 102 is continuously pulled up and controlled, so that the diameter of the seed crystal 102 is constant. Accordingly, a straight body portion 105 is formed. Finally, a silicon single crystal ingot 107 is formed by forming a tail portion 106 by gradually reducing the diameter.
A vitreous silica crucible used for pulling such silicon single crystal generally includes, as shown in FIG. 1, natural fused silica 108, which constitutes the outer portion of the vitreous silica crucible to improve its mechanical strength, and synthetic fused silica 109, which constitutes the inner portion of the vitreous silica crucible to avoid mixing of impurities. Here, the term ‘natural fused silica’ refers to vitreous silica formed of natural silica powder, whereas the term ‘synthetic fused silica’ refers to vitreous silica formed of synthetic silica powder. Generally, a reaction SiO2 (solid)→Si (liquid)+2O occurs at the interface between the synthetic fused silica 109 and the silicon melt 101, and thus the synthetic fused silica 109 is dissolved. While the silicon single crystal is being pulled up, a reaction Si (liquid)+O→SiO (gas) may occur according to conditions, such as a rise in a pulling temperature or a drop in atmospheric pressure. As a result, SiO gas is formed, and, as shown in FIGS. 3(a) and 3(b), the silicon melt 101 may bounce off a surface of the synthetic fused silica 109, and thus melt surface vibration may occur. Furthermore, for convenience of explanation of melt surface vibration, melt surface vibration shown in FIGS. 3(a) and 3(b) is exaggerated.
When such melt surface vibration occurs, the seed crystal 102 may not adhere to a flat melt surface. Furthermore, silicon becomes a silicon poly-crystal while the silicon is being pulled up. In particular, the processes of dipping a seed and forming a shoulder portion in the early stage of the process of pulling the silicon single crystal are easily affected by melt surface vibration, and effects of the melt surface vibration significantly affect quality of a pulled-up silicon single crystal ingot. Therefore, there is demand for a technique for suppressing melt surface vibration of silicon melt during the processes.
JP-A-2004-250304 discloses a technique for adjusting content rate of bubbles in the inner-circumferential surface of a vitreous silica crucible near a melt surface during pulling-up start to be within a predetermined range to suppress melt surface vibration of silicon melt filled inside the vitreous silica crucible. The technique is based on a discovery that melt surface vibration of silicon melt at the time of starting to pull silicon is affected by the content rate of bubbles in the inner-circumferential surface of a vitreous silica crucible near a melt surface.
For example, in the case where a vitreous silica crucible contains a large number of bubbles, the vitreous silica is dissolved as the reaction SiO2 (solid)→Si (liquid)+2O proceeds, and thus opened bubbles 201 as shown in FIG. 4 are formed. The opened bubbles 201 may suppress melt surface vibration according to the same mechanism that boiling chips prevent an abrupt boiling phenomenon. However, if vitreous silica contains a large number of bubbles 202, a ratio of the crucible itself with respect to the volume of the vitreous silica crucible substantially decreases, and thus a dissolution speed increases as compared to the case in which no bubbles are formed. As a result, the lifespan of the vitreous silica crucible is reduced. Recently, a crucible with a large diameter is required to pull silicon single crystal with a large diameter, and thus the cost of a vitreous silica crucible is high. Therefore, there is demand for a vitreous silica crucible, which is capable of suppressing the melt surface vibration and also has a long lifespan due to a slow dissolution speed. Furthermore, unopened bubbles just below the inner surface of the surrounding wall of a crucible expand and rupture while the silicon single crystal is being pulled up, and thus silica fragments are mixed into silicon melt. Therefore, there is also demand for improvement in yield of silicon single crystal.